The objective of these studies is to elucidate the mechanisms by which hormones regulate mammary gland development and milk protein gene expression, and to determine how these regulatory mechanisms have deviated in breast cancer. Aim 1 is to study the regulation and function of different C/EBP isoforms during mammary gland development and carcinogenesis. Aim 2 will attempt to elucidate the function of different Stat5 isoforms during mammary gland development and carcinogenesis. Aim 3 will characterize specific NF1 isoforms expressed in the mammary gland. In Aim 4, mechanisms of post transcriptional regulation of milk protein gene expression will be examined. In Aim 5, the role of p53 and apoptosis in mammary carcinogenesis induced by chemical carcinogens and by the overexpression of IGF1 will be studied.